Mismatch repair is a set of enzymological systems, encoded by numerous genes, that detect DNA base pair mismatches and convert them to standard base pairs. In one mode, mismatch repair examines freshly replicated DNA, detects mismatches of mutational origin, determines which is the wrong (progeny strand) base, and converts it to the correct base. In another guise, mismatch repair examines the hybrid regions of newly recombined DNA molecules, detects mismatches and acts to homogenize them in ways as yet poorly understood. Mismatch repair occurs in at least several bacteria, in yeast and in mammalian cells, and is probably ubiquitous. It constitutes a major barrier to spontaneous and induced mutation and to certain kinds of genetic recombination. A general theory has been developed to explain how mismatch repair may protect against illegitimate recombination in mitosis and meiosis. This work is being prepared for publication and the project is completed.